(1) Field of the invention
This invention relates to a heat-shrinkable cover sheet having two branched tongue-like portions on both the ends in the direction of thermal shrinkage, which comprises a heating element formed of a heating electric conductor wire included in the body portion of the sheet and the tongue-like portions thereof. This heat-shrinkable cover sheet is used for covering and protecting the peripheral surface of a joint of a steel pipe or cable, and exhibits improved applicability and covering property.
(2) Description of the Prior Art
As the conventional heat-shrinkable cover sheet for steel pipes or cables, there can be mentioned a heat-shrinkable cover sheet having connecting branched tongue-like portions on both the ends thereof in the direction of thermal shrinkage, such as those disclosed in Japanese Patent Application Laid-Open Specifications No. 13577/79 and No. 162777/79.
In the known heat-shrinkable cover sheet, covering is accomplished by winding the heat-shrinkable cover sheet around a joint of a steel pipe or cable, joining the tongue-like portions on both the ends of the sheet to form a cylinder, heating the sheet entirely from the outside by flames of a gas burner or the like to effect thermal shrinkage and joining and sticking the sheet closely to the peripheral face of the joint of the steel pipe or cable. Accordingly, it is difficult to uniformly heat and thermally shrink the sheet along the entire periphery of the joint from the outside by flames, and hence, it is very difficult to obtain a covering having satisfactory performance and durability.
As means for overcoming this disadvantage by the above-mentioned flame heating, Japanese Laid-open Patent Application No. 28,489/81 discloses a heat-shrinkable cover sheet each of the ends of which has two branched tongue-like portions in the direction of thermal shrinkage, in which a heating element formed of a heating electric conductor wire is included in the body portion of the sheet, the front tongue-like portion on one end of the sheet and the back tongue-like portion on the other end of the sheet.
In this heat-shrinkable cover sheet having tongue-like portions on both the ends and having included therein a heating element formed of a heating electric conductor wire, covering is accomplished by winding the sheet around a joint of a steel pipe or cable, joining and engaging the respective tongue-like portions of the heating element with each other so that the tongue-like portions overlap each other to form a cylinder, and then, applying electricity to the heating element to effect joining by heating of the engaged portions of the tongue-like portions and also to effect thermal shrinkage by heating of the entire sheet. In this case, the engaged area of the tongue-like portions is double as thick as the body portion, and have therein two superposed end portions of the heating element. Thus, when electricity is applied to the heating element, the temperature is elevated at a higher rate in the engaged area of the tongue-like portions than in the body portion. Accordingly, when thermal shrinkage of the entire sheet is conducted after termination of joining of the engaged area, various problems arise. For example, the already engaged area is liable to be broken into pieces owing to fusion of an adhesive applied to the engaged area, and the tongue-like portions per se are liable to be destroyed by high temperatures.